One of the myriad of uses for polypropylene is for the production of BOPP film. BOPP is used to produce both clear and opaque film for numerous packaging applications. To gain wide commercial acceptance for BOPP film applications, though, a given polypropylene resin must provide uniform stretching under typical BOPP processing conditions. Not surprisingly, not all polypropylene resins exhibit favorable behavior under the mechanical and thermal stresses of the BOPP production process. One resin that tolerates BOPP production conditions is high xylene solubles homopolymer. This resin can be fractionated into three components: an isotactic component, a stereoblock component, and an atactic component.
The stereoblock component is crystalline and melts at a significantly lower temperature than the isotactic component. Film processing performance of the resin, as measured by T. M. Long draw stress, is correlated with the amount and quality of the stereoblock component. The stereoblock component is also believed to provide softening that enables solid-phase drawing to occur under the practical draw stresses observed on a BOPP processing line.
In high xylene solubles homopolymers, the stereoblock component is created by introducing defects which disrupt crystallization and provide a lower-melting component. These defects, however, compromise both the amount and the stereo regularity of the isotactic phase, reducing film strength. Traditionally, high stereo defect concentrations also lead to high xylene solubles content in the polymer which considerably narrows the resin manufacturing process window.
There thus exists a long felt, but unmet need in the art for a BOPP grade resin that maintains the processability of the high xylene solubles homopolymer, but exhibits enhanced characteristics when processed into a BOPP film.